The correct answers are: A. The baseball and D. The hitter.
In a scientific investigation, it is important to control the variables that are not being tested, so that the results are reliable and valid. A variable is anything that can change or affect the outcome of an experiment. The variable that is deliberately changed by the experimenter is called the independent variable, and the variable that is measured or observed as a result of the change is called the dependent variable. The variables that are kept constant or unchanged throughout the experiment are called the controlled variables.
In this investigation, the class is testing how the amount of force applied to a baseball by a bat affects the distance traveled by the ball. Therefore, the independent variable is the force applied by the bat, and the dependent variable is the distance traveled by the ball. The class must keep all other variables constant, such as:
- The baseball: The class must use the same baseball for every trial, because different baseballs may have different masses, sizes, shapes, or materials that could affect how they react to the force of the bat and how far they travel.
- The hitter: The class must use the same hitter for every trial, because different hitters may have different skills, strengths, techniques, or preferences that could affect how they swing the bat and how much force they apply to the ball.
- Other possible controlled variables: The class may also need to control other variables that could affect the experiment, such as the type of bat used, the angle of the bat swing, the weather conditions, or the location of the experiment.
The class does not need to keep the following variables constant:
- The force applied by the bat: This is the independent variable that is being changed on purpose to test its effect on the distance traveled by the ball. The class may use different methods to apply different amounts of force to the ball, such as using a machine or changing their swing speed.
- The distance traveled by the ball: This is the dependent variable that is being measured as a result of changing the force applied by the bat. The class may use different tools or methods to measure how far the ball travels after being hit by the bat.
Therefore, the answer is: A. The baseball and D. The hitter.